The present invention relates generally to the removal of moisture and contaminants from flue gas, and in particular to a new and unique method of utilizing a spray tower to regulate the moisture content of a flue gas.
Fossil fuel combustion is recognized as a major contributor to rising concentrations of carbon dioxide in the earth's atmosphere. The continually increasing carbon dioxide concentration has caused many governments and industries to examine the extensive use of fossil fuels and seek to develop new technologies for reducing carbon dioxide emissions. Major sources of carbon dioxide from fossil fuel utilization include coal-fired power plants, natural gas use for both power production and domestic heating and petroleum fuels for transportation. These three sources are believed to attribute to approximately 36%, 22% and 42%, respectively, of carbon dioxide emissions from fossil fuel fired combustion systems. As the use of coal at power plants represents a point source of carbon dioxide emissions, governments and power generators are focused on developing reliable means to control carbon dioxide emissions from these sources.
Current technologies proposed for capturing, transporting, and disposing of carbon dioxide from power generation facilities are energy intensive and expensive. The most mature CO2 technology for capturing and concentrating carbon dioxide is absorption-stripping using various organic amines such as monoethanolamine, used commercially to remove CO2 from raw natural gas. However, application of this technology to power generation facilities results in power losses exceeding 30% of plant output. Accordingly, a need exists amongst governments and industries which rely upon the combustion of coal to meet their energy requirements to develop economically viable carbon dioxide capture technologies for use in coal fired power generation facilities.
An alternative technology that offers the possibility of concentrating carbon dioxide in power plant flue gases with lower energy penalties than that of amine scrubbing is a technology generally known as oxy-combustion. In a normal combustion process a fuel is burned using ambient air containing about 21% oxygen and 78% nitrogen by volume. The carbon in the fuel reacts with the oxygen in the air to produce carbon dioxide and the hydrogen in the fuel reacts with the oxygen to produce water vapor. The gaseous products of combustion, generally known as flue gases, contain 10 to 15% CO2 and 4 to 7% H2O, and the balance comprising mostly of nitrogen. If the oxygen in the air can be separated from the nitrogen before combusting the fossil fuel, the combustion process would produce a flue gas containing much higher CO2 and water vapor concentrations (typically around 3 times higher and possibly upwards of 5 times higher).
Substitution of concentrated oxygen for ambient air is one of the principal features of oxy-combustion. Another principal feature is flue gas recirculation. A schematic of an oxy-combustion boiler is shown in FIG. 1. Coal and oxygen are supplied to a boiler upstream of a particulate collector and a flue gas desulphurization means. The cleaned flue gas is then split into two streams, one recycled back into the combustion process and the other either vented to the atmosphere or conditioned for sequestration.
If an existing boiler were retrofitted with oxy-combustion technology, the furnace is designed to operate within at a prescribed temperature range, generally about 2500° F. to about 3000° F., and the heat transfer to the various parts of the boiler will have to be unaltered by the new oxy-combustion configuration. Yet, the substitution of oxygen for air can cause the combustion process to operate at significantly higher temperatures, sufficiently high in most case to cause irreparable damage to the furnace. Additionally, in retrofit applications, the mass flow of flue gas would be only about one fifth as much as the boiler was designed, causing the heat transfer distribution to the various components of the boiler to change in ways detrimental to the generation of steam for the turbine generator. However, if flue gases from the boiler exhaust were circulated back to the furnace, the oxygen introduced into the combustion system can be diluted with flue gas, such that the overall oxygen concentration of the oxygen enriched flue gas is near that of air.
By limiting the levels of oxygen introduction into the recirculated flue gas, the furnace can to operate within normal design conditions and the flue gas flow rate and temperature profiles through the rest of the steam generating sections can be made to match design conditions. The recirculated flue gas contains principally CO2 and H2O. Injection of oxygen into the recirculated flue gas, creates an oxygen enriched flue gas having an oxygen concentration approximately that of air, permitting the desired effect of oxy-combustion, i.e., the generation of steam for power production wherein a flue gas comprised significantly of carbon dioxide is produced, allowing for improved disposal of emissions, by means such as, but not limited to, sequestration.
Fossil fuels such as coal naturally comprise impurities such as sulfur, mineral matter, and mercury, the emissions of which must be controlled to meet existing and pending regulations. Since the flow rate of flue gas expelled from the oxy-combustion process would be about one fifth as much as that expelled from a conventional combustion process, the size of the dust collectors, desulphurization processes, and mercury capture devices could be potentially much smaller. However, the concentrations of all of these contaminants in a recirculated flue gas returning to the boiler can be up to five times higher than they would be in the exhaust of a conventional combustion process. For example, if the SO2 concentration of a flue gas from a conventional combustion process is 1500 ppm, the SO2 concentration of a recirculated flue gas can be around 7500 ppm. Similar comparisons can be made for particulate matter and mercury.
Sulfuric acid condensation in air heater and ultimately in the wet scrubber can further be exacerbated by the combined increase in SO2 concentration and moisture concentration in the recirculated flue gas. Reduction in the concentration of any of these contaminants can be greatly improved if the contaminant removal device is placed in the recirculation loop. If done, the advantage of equipment size reduction is minimized, however in many applications, such as retrofits for example, where oxy-combustion is added to existing coal fired boilers, the deleterious effects of these contaminants upon furnace and boiler components is significantly reduced.
If a spray tower such as a flue gas desulphurization apparatus is moved into the recirculation loop to control SO2 concentrations returning to the boiler, the moisture concentration of the flue gas is exacerbated. Wet scrubbers normally behave as adiabatic humidifier causing the moisture content of a flue gas passing there through to rise. For example for a typical high sulfur Illinois coal, the water vapor dew point temperature of flue gas can exceed 170 degrees Fahrenheit, making any recirculated flue gas unusable for coal drying in the pulverizer due to excessive moisture. To avoid potentially adverse impacts, a means of drying and reheating the recirculated flue gas if needed allowing the recirculated flue gas to be used in a pulverizer and as primary air for a coal fired burner.
One alternative is to add means to the recirculation loop to remove a significant portion of the moisture with a condensing heat exchanger. If both SO2 and moisture are to be removed from the recirculated flue gas, a condensing heat exchanger could be located downstream of the desulfurization system. A schematic representation of a boiler with a condensing heat exchanger in a flue gas recycle loop is shown in FIG. 2. This arrangement was tested at pilot scale to remove moisture directly from the recirculating flue gas. While effective, the physical size of this condensing heat exchanger necessary proved prohibitive, and raised other concerns such as but not limited to the production of an acidic condensate.
Accordingly, a need exists for a more efficient, reliable, cost-effective means for removing excess moisture from the recirculated flue gas.